In general, in order to manufacture a semiconductor integrated circuit such as an IC or an LSI, various processes including a film-forming process, an oxidation-diffusion process, an etching process or the like are repeatedly carried out to a semiconductor wafer. In order to conduct the various processes, the semiconductor wafer has to be transferred between respective processing units. In the case, as well known, in view of improvement of yield, it is necessary to prevent that particles and/or a natural oxide film may be formed and stuck on a surface of the semiconductor wafer. Then, as requests for further integration and further miniaturization are increased, for the purpose of the wafer transfer, a containing case body tends to be used, which can contain a plurality of wafers and whose inside can be sealed. As this type of containing case body, FOUP (registered trademark) is known in general (for example, JP Laid-Open publication No. 8-279546, JP Laid-Open publication No. 9-306975 and JP Laid-Open publication No. 11-274267). The containing case body has a box case wherein one side end is formed into an opening and the other side end is formed into a substantially semi-cylindrical shape. Supporting parts are provided on an inside wall surface of the box case at substantially the same pitch in a tier-like manner. A peripheral portion of a semiconductor wafer is placed on and supported by each supporting part. Thus, semiconductor wafers can be contained at substantially the same pitch in a tier-like manner. Usually, about 25 or about 13 wafers can be contained in one containing case body.
An open-close lid is attached at the opening of the box case in a removable and attachable manner. When the open-close lid is attached, the box case can maintain an airtight state to a certain extent. For example, the inside of the box case is filled with an inert-gas atmosphere such as a clean air or an N2 gas. At that time, the contained wafers hardly contact with an outside atmosphere.
A locking mechanism is provided in the open-close lid. By releasing the locking mechanism, the open-close lid can be removed off from the opening.
Herein, the containing case body as described above, such as FOUP, is generally used for wafers having a lager diameter, for example wafers having a diameter of 300 mm (12 inches). The containing case body such as FOUP is not applied to wafers having a smaller diameter, for example wafers having a diameter of 200 mm (8 inches) or a diameter of 150 mm (6 inches). Conventionally, for the wafers having a smaller diameter, an open type of cassette, which doesn't have a sealed structure, is used. The wafers are contained in the open type of cassette in a tier-like manner while the wafers are exposed to an atmospheric air.
Under that situation, based on the requests for further integration and further miniaturization of an integrated circuit, for the purpose of preventing particle problems, it has been requested that the wafers having a smaller diameter are transferred by using a hermetically-closed type of containing case body.
In the case, it can be thought that different sizes of containing case bodies are used for respective wafer sizes. However, this may increase facility cost. Thus, in order to avoid the problem, an assistant jig has been proposed, which makes it possible to fix an open type of cassette for wafers having a smaller diameter to the inside of a containing case body for wafers having a larger diameter.
As the assistant jig, for example, a cassette adaptor AM-3004 (registered trademark) made by Micro Tool co. is known. The assistant jig has a base 2 of a semi-elliptic shape, as shown in FIG. 15. Four columns 4 stand up from peripheral portions of the base 2. Upper ends of the four columns 4 support a ceiling plate 6 of a semi-circular-arc shape, whose diameter is substantially the same as that of a 300 mm wafer. At substantially a center portion of the base 2, positioning protrusions 8 are provided for positioning a cassette C for 200 mm wafers as wafers having a smaller diameter. The cassette C is adapted to be positioned and fixed by the positioning protrusions 8.
Wheels 10 to be held having elasticity, for example made of poly-carbonate resin, are provided at peripheral portions of the base 2 and the ceiling plate 6. The wheels 10 to be held are adapted to be fitted in supporting parts of a containing case body for 300 mm wafers (not shown) which are originally provided for supporting peripheral portions of 300 mm wafers. Thus, the whole assistant jig is adapted to be fixed in the containing case body for 300 mm wafers. That is, when the 300 mm wafers are to be contained, the containing case body for 300 mm wafers is used in a state wherein the assistant jig is removed off. Then, when the 200 mm wafers are to be contained, the containing case body for 300 mm wafers is used in a state wherein the assistant jig is fitted therein, so that the cassette C is fixed and used in the assistant jig. Thus, two kinds of wafers having different sizes can be selectively contained in a hermetically-sealed manner by means of one kind of containing case body.
However, the above assistant jig has the following problems. That is, the center of the 200 mm wafers when the 200 mm wafers are contained with the assistant jig and the center of the 300 mm wafers when the 300 mm wafers are contained without the assistant jig are located at substantially the same position in a horizontal plane. That is, when the 200 mm wafers are contained, a larger space is generated between the 200 mm wafers and the open-close lid. As a result, the 200 mm wafers may considerably slide in the cassette C.
In addition, when the wheels 10 to be held are fitted into the supporting parts for the 300 mm wafers, the supporting parts may be damaged. If cracks or the like are formed in the supporting parts, when the supporting parts support the 300 mm wafers, the 300 mm wafers may be damaged. Furthermore, if cracks or the like are formed in the supporting parts, the thin wheels 10 to be held may be broken and disenabled.